This invention relates generally to sockets for high speed components and more particularly to sockets for testing high speed integrated circuits packaged in ball grid array packages.
Integrated circuits (IC) are usually tested after they are packaged into IC packages to ensure that they are functional before they are assembled into final systems. There are problems inherent in devising appropriate test sockets for these packaged ICs, particularly where the ICs are packaged in ball grid array (BGA) packages and designed for high speed use.
BGA package are packages which have an array of spherical contacts on their bottom surface for connection from the IC to a substrate, typically a printed circuit board (PCB). BGA packages are normally used for ICs which require a large number of contacts. The spherical contacts are typically eutectic solder balls which are reflowed during assembly to a PCB to form a connection between the package and the PCB. Before assembly, the spherical contacts are non-compliant. Although there is typically a specification regarding the planarity of the bottom surface of the spherical contacts, some non-planarity is inherent in the manufacturing process. The combination of non-planarity and non-compliance results in difficulty in ensuring that all of the spherical contacts make the required electrical contact with test pads within a test socket during testing.
One solution which has been used to overcome the contact problem with BGA packages is to use a test socket which has spring loaded pins rather than test pads. The spring loaded pins compensate for any lack of planarity of the spherical contacts and thereby ensure that electrical contact is made with all of the spherical contacts for testing. The problem with the use of spring loaded pins for high speed applications is that the testing of the package in a socket with spring loaded pins does not adequately represent the configuration of the package during use. In particular, the spring loaded pins increase the length of the circuit path. For the testing of ICs designed for high speed applications, the increase in the length of the circuit path is unacceptable because it means that the ICs can not be properly tested at high speeds.
Another problem with the testing of BGA packages and other packages with a large number of contacts is the means by which the package contacts are brought into and maintained in electrical contact with the electrical connections of the test socket. Typically, a lid is used. The lid is pivoted around a hinge which lies along one edge of the socket. The lid is rotated into contact with the package and a lever arrangement is used to apply downward force to the lid and thereby to the package. The geometry of this arrangement means that the force applied to the package by the lid has both a vertical and a horizontal component and that force is not applied to all of the contacts at the same time but instead is applied first to those contacts closest to the hinge. The result is that the horizontal force component may cause horizontal movement or deformation of the contacts of the package such that they will not correctly align with the electrical connectors of the test socket when the lid is closed.
The present invention is directed to an improved socket for testing ICs and, in particular, high speed ICs in BGA packages. The socket uses resilient conductive test pads positioned on a substrate, or conductive test pads positioned on a resilient substrate, in an array to match an array of spherical contacts on the bottom of a BGA package. The BGA package may be aligned with the test pads by the use of holes centered on the test pads into which the spherical contacts seat themselves. The holes may also be used to interconnect conductive signal paths on layers of the substrate or to seat the package without the presence of the test pads.
The present invention also contemplates an improved means of holding an IC package in position in the test socket. In particular, the test socket is provided with two flexible seals, one outside the other, on a support surface. The flexible seals, together with the support surface and a bottom of a socket lid define an enclosed cavity. A vacuum is applied to the enclosed cavity which compresses the flexible seals and pulls the socket lid towards the test pads. The bottom surface of the socket lid is adapted to apply downward force to the IC package to force the IC package leads into contact with the test pads when the socket lid moves towards the test pads.
Advantageously, the use of resilient conductive test pads or substrate compensates for any non-planarity in the spherical contacts.
Also advantageously, the location of the test pads directly on the test substrate minimizes the additional length of signal path introduced by the test socket.
A further advantage of the present invention is that the use of the holes to align the BGA package to substrate minimizes the hardware required for the test socket.
Another advantage of the present invention is that the leads of the IC package are brought into contact with the test pads using only a vertically downward force.
A further advantage of the present invention is that the socket lid is separate from the remainder of the socket and so can be moved out of the way when an IC package is inserted into the socket.
Another advantage of the present invention is that the vacuum may be applied in a pulsing manner thereby scrubbing the leads of a non-BGA IC package or, in the case of a BGA package, helping to vibrate the leads of a BGA package into the holes.
Other aspects and features of the invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.